The present invention pertains to Bipolar Junction Transistors (BJTs), and more particularly to Double Heterojunction Bipolar Transistors (DHBTs).
InP is known to have advantages as a collector material. It has high electron saturation velocity, resulting in short collector transit times, and can support high breakdown voltages due to its large bandgap and breakdown fields. It also has better thermal conductivity than many materials, such as InAlAs and InGaAs (lattice-matched to InP), which enhances its heat dissipation. Furthermore, when highly doped it permits fabrication of non-alloyed ohmic contacts.
DHBTs having InP collectors are known. For example, William Liu in the Handbook of III-V Heterojunction Bipolar Transistors, 1998 John Wiley and Sons, describes a DHBT having an InP collector and a base of InGaAs. The difficulty of such a device is that InGaAs, at least if lattice-matched to InP, has a conduction band energy minimum which is lower than the conduction band energy minimum of InP. This results in electrons accumulating on the base side of the base-collector heterojunction.
DHBTs having an InP collector and a base of GaAsSb are also known. For example, U.S. Pat. No. 4,821,082 to Frank, et al., describes a DHBT having collector and emitter of InP, and a base of GaAs0.53Sb0.47. U.S. Pat. No. 5,349,201 to Stanchina, et al. and Publication EPO 715357 A1 to McDermott also describe DHBTs having collector and emitter of InP, and base of GaAsSb. The problem with this approach is that GaAS0.53Sb0.47 (GaAsSb lattice-matched to InP) has a conduction band energy minimum which is approximately 0.12 eV higher than that of InP. Although the combination of a base of GaAsSb, lattice-matched to a collector of InP, eliminates the problem of electron accumulation at the base-collector interface of the DHBT, it instead causes injection of high energy electrons into the collector from the base, because the electrons gain energy equal to the conduction band offset at the base-collector junction. Such injection of high energy electrons into the collector reduces base-collector breakdown voltage by facilitating impact ionization. If the base GaAsSb is instead formulated for conduction band energy matching to InP, then the material will be in tensile strain with respect to InP, which can degrade device reliability.
It is well known that the gain of a transistor is improved when the valence band energy level of the emitter is significantly lower than that of the base.
Accordingly, there is a need for a BJT device, and for a method of making such a device, incorporating the advantages offered by an InP collector, and wherein the conduction band energy minimum of the base is closely matched to that of the collector in order to avoid both accumulation of electrons at the base-collector junction (when the base conduction band energy minimum is too low), and injection of high-energy electrons from the base into the collector (when the base conduction band energy minimum is too high). Such a device preferably has little or no lattice mismatch, and has a base with valence band energy minimum significantly higher than that of the emitter.
It is an object of the present invention to provide a device, and a method of making the same, which fills this need.
The present invention employs a base which either is, or else mimics, a quaternary compound InxGa1xe2x88x92xAsySb1xe2x88x92y, in which x and y are preferably selected such that the compound is lattice-matched to InP. Lattice matching can be achieved for certain values of x and y within the range from (x,y)=(0.53, 1) to (x,y)=(0, 0.5). The most preferred formulation within this range is that in which the conduction band minimum energy of the compound is from 0 to 10 meV higher than the conduction band energy minimum of InP. InxGa1xe2x88x92xAsySb1xe2x88x92y achieves such an optimum conduction band energy level when x is about 0.16 and y is about 0.65. The base may also be formulated to be somewhat compressively strained to InP, but if so then it is preferably thinner than critical thickness.
The base may be a desired formulation of monolithic InGaAsSb, or may be fabricated as a superlattice having alternating layers of ternary compounds such as InGaAs and InGaSb. The superlattice materials are preferably selected such that the average proportions of the elements in the overall superlattice structure are (approximately) the same as for the desired formulation of InGaAsSb. If the periods of the superlattice are sufficiently thin, the superlattice will mimic the properties of the corresponding monolithic quaternary compound of InGaAsSb.
Thin superlattice periods also help reduce disruption of the superlattice crystal structure due to strain. If a particular quaternary compound is lattice-matched to InP, then a superlattice having the same average formulation will generally have no net strain to InP. The sublayers of each period of such a superlattice will generally be strained, but the sum of strains over each period will generally equal zero.
InP is employed in the collector. The emitter of a device according to the present invention is preferably InP or InAlAs lattice-matched to InP. Each has advantages, and they differ largely in convenience for fabrication. For embodiments employing an InAlAs emitter, it is desirable to grade the base-emitter junction, for example by continuous composition change or using a chirped superlattice, to shift the discontinuity of conduction band energy minimum into the valence band. Delta doping techniques are helpful to improve the grading effects, and also to permit a wider depletion region.